This application claims priority to Patent Application No. 2004-129909 filed in Japan on 26 Apr. 2004, the content of which is hereby incorporated herein by reference in its entirety.
The present invention pertains to a copier, optical printer, facsimile machine, or other such image forming apparatus making use of electrophotography, and to a method for detecting toner level in the context of such an image forming apparatus. In particular, the present invention pertains to an improvement for detecting, with high precision, when toner level has become low (hereinafter referred to as “low toner level”).
Conventionally, in a typical electrophotographic image forming apparatus, toner might be delivered from a toner hopper storing toner to a developer storage container, and toner within such a developer storage container might be transported onto a photosensitive drum for formation of prescribed toner images.
Furthermore, with respect to the timing with which toner is delivered from this toner hopper to the developer storage container, taking for example the case of an image forming apparatus employing two-component developer comprising toner and carrier, a toner sensor comprising a magnetic permeability sensor might be disposed within the developer storage container, it being determined that the amount of toner within the developer storage container is insufficient and toner being supplied from the toner hopper to the developer storage container when the magnetic permeability detected by this toner sensor is greater than or equal to a prescribed value (i.e., magnetic permeability increases as the amount of toner decreases). This makes it possible to maintain more or less constant concentration of toner within the developer storage container.
Moreover, in the event that a state in which concentration of toner is low (i.e., a state in which magnetic permeability is greater than or equal to a prescribed value) persists for a prescribed time or longer despite having carried out supply of toner pursuant to the aforementioned control, it being determined that there is no longer any toner within the toner hopper, a depleted toner condition might be declared and the user might be prompted to replace the toner hopper or fill same with toner.
However, where this method is employed, because reporting of the depleted toner condition occurs in abrupt fashion, a situation might occur in which the apparatus cannot be used during the time while the user is getting a replacement toner hopper ready. In particular, when no toner hopper is readily available to the user, it is possible for a situation to occur in which the apparatus cannot be used for a long period of time.
Japanese Patent Application Publication Kokai No. H2-280176 (1990) (hereinafter “Patent Reference No. 1”), Japanese Patent Application Publication Kokai No. H9-197797 (1997) (hereinafter “Patent Reference No. 2”), Japanese Patent Application Publication Kokai No. 2000-338767 (hereinafter “Patent Reference No. 3”), and Japanese Patent Application Publication Kokai No. H10-207213 (1998) (hereinafter “Patent Reference No. 4”) have therefore proposed detecting when the amount of toner is low in the form of a low toner level condition at a stage prior to when there would be no more toner within the toner hopper, this being reported to the user so as to give the user additional time to have a toner hopper ready before occurrence of a depleted toner condition.
Patent Reference No. 1 discloses a method combining use of a sensor detecting optical density and a sensor detecting magnetic permeability of developer. This is such that toner concentration within developer is controlled so as to be constant by means of magnetic permeability detection while optical density of a specified pattern on the photosensitive body is at the same time detected to maintain final image density, accurate detection of low toner level and depleted toner conditions as well as display of warnings with respect thereto being carried out based on the two sensors.
For proper supply of toner that is neither too much nor too little in light of changes in fractional image area as well as variation in ability to supply toner across different apparatuses due to variation in ability to deliver toner, Patent Reference No. 2 proposes adjustment of supply amount in correspondence to a maximum supply ratio; i.e., by means of a ratio between toner supply member drive time existing upon obtaining an image density at a photosensitive body detected by a photosensor and toner supply member drive time existing when supplying a maximum amount of toner.
Patent Reference No. 3 discloses a method in which a low toner level condition is declared in the event that a toner supply drive member is driven for a prescribed time or longer.
Patent Reference No. 4 discloses carrying out determination of existence of a low toner level condition based on relative number of toner supply iterations and/or detection of decrease in toner concentration.
At the aforementioned Patent Reference No. 1, the fact that a plurality of sensors are employed makes for complicated structure and/or increased cost. And what is more, to the extent that a plurality of sensors are provided it will also be true that there will be an increased number of factors contributing to detection of false positives, making it difficult to adequately ensure reliable detection of the low toner level condition.
Because they make it possible to report a low toner level condition by means of an inexpensive and simple structure, Patent Reference Nos. 2 through 4 permit elimination of the problems at Patent Reference No. 1. However, because Patent Reference No. 2 is such that the low toner level condition is detected based on image density at the photosensitive body, it is impossible to carry out detection in a manner such as would exclude other factors (factors other than toner level) causing changes in image density at the photosensitive body. That is, as it will not be possible to detect toner level with high precision when image density at the photosensitive body changes due to the influence of photosensitive body deterioration, temperature, or humidity, it will be difficult with the method of this Patent Reference No. 2 to adequately ensure reliable detection of the low toner level condition.
Furthermore, with the method of Patent Reference No. 3, because variation in amount supplied per unit time by respective toner supply drive members is not small, existence of this variation in ability to deliver toner produces error in detection of depleted toner and low toner level conditions. As a result, it is possible for a problematic situation to occur in which a hopper replacement request is reported despite the fact that a sufficient amount of toner remains within the hopper; and conversely, it is possible for a problematic situation to occur in which a hopper replacement request is not reported despite the fact that image formation is jeopardized due to absence of toner.
Moreover, with the method of Patent Reference No. 4, not only does variation in ability to deliver toner constitute a factor contributing to error as was the case above, but the magnitude of print coverage can also be responsible for an additional cause of error, it being possible that a low toner level condition will be mistakenly reported during execution of printing operations when print coverage is high.